Sensors for detecting relative in-plane or sliding displacement between adjacent parallel surfaces are well known in the art. Optical applications for such displacement sensors include precision alignment and/or positioning of optical elements, where high precision is required. An example of a segmented optical element is the primary mirror of the Keck telescope in Hawaii, a description of which can be found in the Keck Observatory Report number 90 published by the Keck Observatory, Lawrence Livermore Laboratories, Berkely, Calif. For example, in aligning the mirror segments of the Keck telescope, displacement sensors must indicate with subnanometer resolution when the reflective surfaces of adjacent segments are aligned.
Other optical applications place more stringent demands on these sensors. Adaptive optical elements used with high energy lasers (HEL), such as the mirror disclosed and claimed in the commonly owned co-pending U.S. patent application entitled "Extendable Large Aperture Phased Array Mirror System", Ser. No. 114,540 employ large numbers of small segments to alter an optic surface to generate a conjugate wavefront. Displacement sensors indicate segment mirror surface tilt as well as displacement, and therefore must have an extremely stable zero point, have a wide dynamic range and be especially insensitive to environmental (e.g. thermal) changes. The very large number of segments mandates the use of thousands of displacement sensors. Consequently, sensors that have complex electronics or which are not amenable to minaturization and batch fabrication have limited utility. Moreover, sensors used with these phased array mirrors must display low sensitivity to changes in lateral gap between adjacent segments.
Several displacement sensor technologies have been developed for high precision applications. Included are optical sensors which typically comprise an emitter and detector located respective on opposing surfaces and which usually employ a digital grating. These sensors lack the subnanometer resolution and extremely stable zero point operation required for segmented adaptive optics elements.
Capacitive displacement sensors have been successfully used in certain applications, since they can be extremely stable and provide subnanometer resolution in conjunction with low complexity electronics. However, the individual sensors are comprised of a plurality of carefully fabricated and aligned discrete electromechanical parts and are therefore not practical for mirrors or other optical elements comprised of tens or hundreds of thousands of individual segments.
It would be advantageous to have a displacement sensor acceptable to miniaturization and batch fabrication, requiring no gap-bridging part, capable of subnanometer resolution with a wide dynamic range, and using very simple electronics. The present invention is directed towards such a sensor.